The invention relates to an abradable layer for labyrinth seals of a turbo-engine, particularly a gas turbine, made of a layered composite material with a core and a shell and embedments, and to a process for the manufacturing of this abradable layer.
Abradable layers for turbo-engines are known from the European patent Document EP-OS 0 166 940. The abradable layers disclosed there have the purpose of keeping the radial gap between the rotor blades and the housing as small as possible. When such abradable layers for labyrinth seals having, for example, labyrinth peaks, are used on the rotor shaft and the abradable layer on the housing, the danger of self-ignition of the abradable layer exists. This danger exists because, at the inlet, the labyrinth peaks are in a much more intensive frictional contact with the abradable layer than the individual blade tips of a rotor with the housing abradable layer.
For this reason, the abradable layers are not, as disclosed in the European Patent Document EP-OS 0 166 940 made of a core material of graphite or ceramics, but of a metal felt or metal fabric which better carries off heat, without any sheathings or embedments.
Despite this difference, engine failures occur as a result of the self-ignition of abradable layers made of metal felt or metal fabric on labyrinth seals. In this case, there is a local overheating of the metal felt or metal fabric as a result of an intensive frictional contact with the labyrinth peaks. As a result, there are exothermal reactions between condensed hot atmospheric oxygen (0.4 to 1 MPa at 280.degree. to 600.degree. C.) and the large specific oxidizable metal surface of the metal felt or metal fabric. In this case, the combustion rate is higher than the carrying-off of heat so that the seal construction is destroyed in an explosive manner. In this case, there may also be a partial welding-together of the abradable layer material with the rotor.
It is an object of the present invention to provide an abradable layer of the above-mentioned type with improved starting and grind-in characteristics in the case of which an exothermal reaction is checked, and the thermal conduction characteristics are improved.
According to the present invention, this object is achieved in that the core is made of a felted or three-dimensionally crosslinked fiber body of iron, nickel or cobalt base alloys. The shell is made of one or several precious-metal alloys. Each metal core is surrounded by the shell material. The embedments consist of oxidation-resistant sliding materials from the group of oxides, carbides or nitrides with a hexagonal crystal lattice layer structure.
This solution has the advantage that the core made of an iron, nickel or cobalt alloy furnishes the required stability in order to form an abradable layer that corresponds to the shape. The precious metal shell has a higher resistance to oxidation than the dimensionally stable core material and limits and prevents the propagation of an exothermal reaction through the oxidation-resistant barrier layer. The fiber body starts to glow locally and also partially starts to melt so that the seal is ground in, but an explosion-type propagation of the combustion is prevented. At the same time, the precious metal shell increases the carrying-off of heat because precious metals, as a result of their higher electron mobility, exhibit a higher heat conduction than non-precious metals.
In the case of conventional abradable layers for blade devices, embedments have the task of grinding in the metallic blade tips and therefore consist of corundum or other hard grinding particles. In the case of labyrinth seals, such grinding particles would wear away the metallic peaks of the labyrinth seals and would increase the leakage rate. The embedments according to the invention, consisting of sliding materials from the group of oxides, carbides or nitrides with a hexagonal crystal lattice layer structure, have the advantage that the hexagonal crystal lattice layers, because they are not hexagonally densest ball packings, can be displaced in a sliding manner and thus do not abrasively stress the peaks of the labyrinth seals.
In the case of a preferred development of the invention, the fiber body is made of metal wire, metal fiber or metal shavings. Depending on the type of cross linkage, these may be long or short. According to the shape and type of the fiber, different manufacturing processes are advantageously used for forming a fiber body. From filament fibers having a diameter of approximately 1 to 50 .mu.m, two-dimensional cross-layer-wound cylindrical layers can be produced which are crosslinked by diffusion contact sintering or homogenizing at the crossing points of the fiber winding to form a fiber body. However, the filament fibers are also suitable for producing needle felts or for forming three-dimensional knitted or woven fabrics. Long and short fiber shavings may be processed into needle felt which is particularly suitable for a three-dimensional crosslinking by diffusion contact sintering or homogenizing to form a fiber body.
Metal fibers, filaments or wires which are electro-deposited on cotton fibers or carbon fibers may be processed into fiber bodies in the same manner as metal shavings.
Iron or iron base alloys, nickel or nickel base alloys or cobalt or cobalt base alloys are used as the material for metal wires and metal fibers or metal shavings.
It is a significant advantage of metal wire, metal fibers or metal shavings that the fiber body has a high initial porosity so that a large pore volume is available for embedments.
In another preferred development of the invention, the fiber body has flat knitted, woven, knotted or wound, two-dimensional fiber layers which, in the third dimension, are crosslinked by diffusion contact sintering or homogenizing or by knitting, crocheting, quilting or sewing. This has the advantage that, during the grinding-in of the labyrinth peaks, the fiber body cannot be denuded in layers so that no large-surface foreign particles peel off during the running-in and therefore possibly impair the function of a power unit.
As the precious metal for the cladding of the fiber core, platinum, rhodium, gold or alloys of these elements are preferably used. These oxidation-stable and heat-conducting metals have the advantage that they can be entered into the fiber body for the cladding of the metal wires, metal fibers or metal shavings in a very uniform and cost-effective manner.
A preferred embodiment is made of particles of hexagonal boron nitride. In addition, the hexagonal boron nitride improves the friction behavior and grazing behavior when it is entered as an embedment into the abradable layer before, during, or after the shaping of the abradable layer. Pyrolytically deposited .alpha.-boron nitride has a hexagonal crystal lattice layer structure and is not only extremely oxidation stable, but also has a coefficient of friction like that of graphite. It does not react with most molten metals, is not wetted and is as soft as talcum. Since it has considerable caloric conductibility, it reduces the rise of the temperature of the abradable layer when the pores of the fiber body are filled with .alpha.-boron nitride. The temperature rise is also reduced by the decrease of the coefficient of friction of the abradable layer as a result of the soft boron nitride.
The object of providing a process for the manufacturing of the abradable layer according to the above-mentioned type is achieved in that first the core material is coated with one or several previous metals or their alloys. Then, the spaces or pores of the fiber body are completely or partially filled with embedments so that a crude fiber body is created which is then processed into an abradable layer blank and is finally pressed to form an abradable layer.
This sequence of process steps has the advantage that, before the abradable layer is completed, quality-ensuring intermediate products are manufactured, such as a coated core material after a precious metal coating step; a crude fiber body after a charging operation for embedments; an abradable layer blank after a processing and shaping step; and finally the abradable layer is manufactured after a pressing operation. Each intermediate product may be subjected to a separate quality inspection in order to advantageously ensure the quality of the end product. Thus, after the precious metal coating step, this process permits the determination of the completeness of the cladding of the core material and the thickness of the precious-metal cladding. After the embedding of the sliding materials, the filling ratio of the pores and the remaining pore volume can be examined on the crude fiber body. After the processing and forming step, the dimensional accuracy of the abradable layer blank can be measured, and the amount of shrinkage is determined for a compressing step by pressing, and thus the dimensional accuracy of the abradable layer is ensured. After a quality-ensuring step, a reworking of the pressed abradable layer is also possible.
The manufacturing has the additional advantage that it is extremely economical because commercially available qualities can be used as the core material which are shown in Table 1 by their trade name and chemical composition and which are refined by means of the process according to the present invention.
TABLE 1 __________________________________________________________________________ Typical Chemical Composition of Commercially Available Metal Fiber Alloys as Core Material Tradename Ni Co Cr Mo W Fe C Si Mn Al Y Ti __________________________________________________________________________ Hastelloy Basis 1-2 20-20 8-10 0.5-1 16-20 &lt;0.15 0.5-1 0.5-1 Hastelloy 18-25 Basis 18-25 12-15 1-2 &lt;0.15 0.1-0.3 0.5-1 188 FeNICrALY 22-28 0.3-1 16-22 Basis &lt;0.1 &lt;0.2 7-12 &lt;0.1 Brunslloy 53 X Cr--Ni 8-12 16-20 1-3 Basis &lt;0.1 0.5-1 &lt;0.5 Stahl US- Typ304/404 __________________________________________________________________________
Another preferred implementation of the process consists of the fact that first the core material is processed to form an uncoated crude fiber body, is then pressed or diffusion-connected to form an abradable layer blank, and subsequently a precious-metal coating takes place by electroplating, chemical vapor depositing or plasma depositing, and finally embedments are entered.
This process sequence has the advantage that, before the precious-metal coating, a relatively compact crude fiber body exists already whose large number of fiber crossing points is fixed by the precious-metal coating, whereby an improved three-dimensional breaking strength is achieved.
Despite the compressing, the grain size of the embedding material is still small with respect to the pore size of the compacted crude fiber body so that a sufficient concentration of the embedding material can be achieved.
For the charging of hexagonal boron nitride as the embedding material, preferably after the coating of the core material with precious metals, an impregnating of the fiber body is carried out by a slurry of particles made of hexagonal boron nitride in a carrier liquid. After the evaporating or air drying of the carrier liquid, hexagonal boron nitride particles remain in the pores of the fiber body. The resulting crude fiber body can then be processed to form an abradable layer blank and can finally be compressed to form an abradable layer by pressing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.